This invention relates to fiducial markers for frameless stereotaxis and magnetic surgery.
Stereotaxic neurosurgery is evolving rapidly due in large part to the development of fast, powerful computers and innovative imaging and interactive image-guided neurosurgical techniques. The ability of the modern computer to quickly execute complex geometric calculations is being harnessed to provide spatial information which in the past could only be obtained by using precisely machined mechanical devices attached to rigid frames.
One of the most active areas of development in stereotaxic neurosurgery today is frameless stereotaxis (FS) and related imaging technologies. The fundamental concept behind the various FS systems is to provide the surgeon with precise intracranial localization information without the use of a conventional fixed head frame.
Frameless stereotaxic treatment methods require that the position of the skull be accurately registered during surgery. A widely used and efficient approach to achieving this objective was to use a number of clearly defined points on the skull as registration markers. These points must be clearly visible on the pre-operative MRI or CT scan and during the treatment phase of the procedure. The ideal registration method would be safe, noninvasive and highly accurate.
The process of intraoperative localization is carried out as follows. First, the patient undergoes a computer tomography (CT) or magnetic resonance imaging (MRI) with fiducial markers in place. These fiducial markers must be in fixed positions relative to the skull and are made of materials that are clearly visible on the preoperative imaging studies. During surgery, the patient's head is secured in place so that it does not move relative to a frameless position sensing system. A variety of sensing systems are currently in use, mechanical translation devices, ultrasonic sound detectors, and infrared light detectors, for example, to precisely determine the location of a hand-held locating wand within a three-dimensional volume. The locations of the patient's brain structures within the three-dimensional volume are determined by first delineating the fiducial marker locations with the localizing wand, and then touching the particular intracranial point in question. After the fiducial point locations have been logged in, the FS computer determines where the wand tip is relative to the preoperative imaging study and displays this information to the surgeon.
To make clinical use of preoperative MRI or CT data, the frameless stereotaxis computer must be given accurate information concerning the patient's head position during both the preoperative radiologic study and the surgical or radiation treatment. Traditional rigid frames attached to the skull subserve this function adequately (e.g. BRW or Lexel headframes). With the evolution of frameless stereotaxis methods, however, these well proven spatial reference systems have excess structural capacity that is unnecessary and cumbersome.
Fiducial skull markers play a critical role in this process. They must be well visualized and remain stationary relative to the skull from the time the imaging study is obtained to the time surgery is conducted. One approach used is to screw the markers into the skull. This can be carried out under local anesthesia as a minor operation. This approach has the advantage of great precision and the disadvantages of being an invasive procedure and having a time limitation in their use. Infection risk increases with chronic insertion. The other widely used method is to glue markers to the scalp. This approach has the advantage of being noninvasive, and the disadvantage of a relatively unstable mechanical purchase on the skin. The markers might become dislodged during the time interval between the imaging study and surgery.
Numerous "frameless" registration methods have been developed to encode head position information more efficiently. A fundamental requirement for all these methods is that at least three discrete points positioned on or around the patient's head must be visible on the preoperative imaging studies and during the treatment phase. Noninvasive methods, such as identifying skull surface features or gluing fiducial markers to the scalp, represent a significant improvement in patient comfort but are not as consistently accurate as methods using rigid points of skull fixation. The greatest degree of accuracy has been achieved using markers that are screwed into the skull. Although these markers can be inserted quickly with minimal inconvenience to the patient, they are invasive and cannot be left in place for extended periods of time.
As defined in a recent review of medical image registration techniques, "extrinsic" point methods means man-made fiducial objects are attached to the skull. A wide range of noninvasive extrinsic marker methods have been used, including skin staples and spheres glued to the scalp. Methods based on skin fixation have the advantages of safety and ease of use. Skin is not rigidly attached to the skull, however, and is thus susceptible to slight shifts in position. Also, spheres glued to the scalp may occasionally be dislodged and are not designed for long-term fixation. Markers screwed into the skull are highly accurate and mechanically stable. Because of their invasive nature, however, there is a small surgical risk associated with their use. Patients experience mild discomfort during the insertion procedure, and there is no opportunity for convenient reapplication during serial studies spanning several weeks to months.
A need exists for a new skull fiducial marker system which can be used for frameless stereotaxic and magnetic neurosurgery, serial and cross-modality volumetric diagnostic imaging, as well as focused radiation therapy. A need exists for systems which fix rigidly to the skull, are simple to use, are noninvasive, are reattachable over long periods of time, and have submillimetric accuracy.